1. Field of the Invention
The present invention relates to a magneto-optical recording medium and a reproducing method thereon. In particular, the present invention relates to a magneto-optical recording medium and a reproducing method thereon which are suitable for high density recording and which make it possible to perform reproduction by magnifying a minute recording magnetic domain which is extremely smaller than a reproducing light spot.
2. Description of Related Art
The magneto-optical recording medium is a highly reliable recording medium having a large storage capacity on which information is rewritable. Therefore, the magneto-optical recording medium begins to be practically used as a computer memory and the like. However, a technique for performing recording and reproduction at a higher density is demanded in view of the increase in amount of information and the advance of the apparatus to acquire a compact size. In order to record information on the magneto-optical recording medium, a recording system based on the magnetic field modulation is used, in which a magnetic field having a polarity corresponding to a recording signal is applied to a portion at which the temperature is raised, while irradiating the magneto-optical recording medium with a laser beam. This system makes it possible to perform overwrite recording, in which high density recording has been achieved. For example, recording has been achieved with a shortest mark length of 0.15 xcexcm. A recording system based on the optical modulation has been also practically used, in which recording is performed by radiating a power-modulated light beam corresponding to a recording signal while applying a constant magnetic field.
When it is intended to reproduce information from a recording mark having been recorded at a high density, a problem arises concerning the optical reproducing resolving power which is determined by a spot diameter of a reproducing light beam. For example, it is impossible to perform reproduction while distinguishing a minute mark having a magnetic domain length of 0.15 xcexcm by using a reproducing light beam having a spot diameter of 1 xcexcm. In order to eliminate the restriction for the reproducing resolving power resulting from the optical spot diameter of the reproducing light beam as described above, one approach has been suggested concerning the magnetically induced super resolution technique (MSR) as described, for example, in Journal of Magnetic Society of Japan, Vol. 17, Supplement No. S1, p. 201 (1993). This technique utilizes the occurrence of the temperature distribution over a magnetic film included in a reproducing light beam spot when a magneto-optical recording medium is irradiated with a reproducing light beam. A magnetic mask is generated in the spot so that the effective spot diameter, which contributes to signal reproduction, is reduced. The use of this technique makes it possible to improve the reproducing resolving power without reducing the actual spot diameter of the reproducing light beam. However, in the case of this technique, since the effective spot diameter is decreased by means of the magnetic mask, the amount of light which contributes to the reproduction output is decreased, and the reproduction C/N is lowered to that extent. As a result, it is difficult to obtain sufficient C/N.
Japanese Laid-Open Patent Publication No. 1-143041 discloses a method for performing reproduction on a magneto-optical recording medium comprising a first magnetic film, a second magnetic film, and a third magnetic film which are magnetically coupled to one another at room temperature. Assuming that the first, second, and third magnetic films have Curie temperatures of TC1, TC2, and TC3 respectively, there are given TC2 greater than room temperature and TC2 less than TC1, TC3. The coercive force HC1 of the first magnetic film is sufficiently small in the vicinity of the Curie temperature TC2 of the second magnetic film. The coercive force HC3 of the third magnetic film is sufficiently larger than a required magnetic field in a temperature range from room temperature to a required temperature TPB which is higher than TC2. The magneto-optical recording medium is used to perform reproduction while magnifying the recording magnetic domain in the first magnetic film. This method utilizes the increase in temperature of the medium when the reproducing light beam is radiated so that the magnetic coupling between the first and third magnetic films is intercepted. In this state, the magnetic domain in the first magnetic film is magnified by using the externally applied magnetic field and the diamagnetic field acting on the recording magnetic domain. It is noted that this technique uses the second magnetic film in which the Curie temperature is set to be lower than the temperature of the readout portion during reproduction. However, the present invention does not use any magnetic film having such a magnetic characteristic.
Japanese Laid-Open Patent Publication No. 8-7350 discloses a magneto-optical recording medium comprising a reproducing layer and a recording layer on a substrate, on which reproduction can be performed while magnifying a magnetic domain in the recording layer during the reproduction. When the magneto-optical recording medium is subjected to reproduction, an alternating magnetic field is used as a reproducing magnetic field to alternately apply a magnetic field in a direction to magnify the magnetic domain and a magnetic field in the opposite direction. Thus, the magnetic domain is magnified and reduced for each of the magnetic domains.
The present inventors have disclosed, in International Publication WO 97/22969, a method for performing reproduction on a magneto-optical recording medium, in which a reproducing light beam is radiated onto the magneto-optical recording medium having a magneto-optical recording film which is a perpendicularly magnetizable film at a temperature not less than room temperature to detect the magnitude of the magneto-optical effect so that a recorded signal is reproduced.
The magneto-optical recording medium to be used is a magneto-optical recording medium comprising, on the magneto-optical recording film, an auxiliary magnetic film which causes transition from an in-plane magnetizable film to a perpendicularly magnetizable film when the temperature exceeds a critical temperature, with a non-magnetic film interposed therebetween. The magneto-optical recording film and the auxiliary magnetic film satisfy a relationship of room temperature less than TCR less than TCO, TC provided that the magneto-optical recording film and the auxiliary magnetic film have Curie temperatures of TCO and TC respectively, and the critical temperature of the auxiliary magnetic film is TCR. The recording signal is reproduced by irradiating the magneto-optical recording medium with the reproducing light beam which is power-modulated at the same cycle as that of a reproducing clock or at a cycle created by the multiplication of an integer and the reproduction clock. In this reproducing method having the foregoing feature, the reproducing light beam is modulated to have reproducing light powers of Pr1 and Pr2 at the same cycle as that of the reproducing clock or at the cycle created by the multiplication of an integer and the reproducing clock. This patent document discloses that one of the reproducing light powers of Pr1 and Pr2 is a power to cause magnification of the magnetic domain in the auxiliary magnetic film. The principle of the reproducing method will be explained by using a schematic diagram concerning the reproducing method shown in FIG. 19. In this reproducing method, as conceptually shown in FIGS. 6A and 6B, a magneto-optical recording medium is used, which has a structure comprising, on a recording layer 10, an auxiliary magnetic layer 8 with a non-magnetic layer 9 intervening therebetween. At first, a predetermined recording pattern as shown in FIG. 19(a) is recorded on the second type magneto-optical recording medium as the magneto-optical recording medium by using, for example, the optical modulation recording system. In FIG. 19(a), the recording mark is recorded at a shortest mark pitch DP, and the recording mark length DL is set to give DL=DP/2. Upon reproduction, a pulse laser beam, which is modulated to have two kinds of reproducing powers Pr2, Pr1, is used as the reproducing laser beam to be radiated so that the cycle which synchronized with the recording mark position is DP, and the light emission width of the high power Pr2 is DL as shown in FIG. 19(b). The light beam having the low reproducing power Pr1 is always radiated in an erasing state (onto portions at which no recording mark exists), and the light beam having the high reproducing power Pr2 is radiated in a recording state (onto portions at which the recording mark exists) and in the erasing state.
FIG. 19(c) illustrate a reproduced signal waveform obtained by radiating the reproducing pulse laser as shown in FIG. 19(b). On the other hand, FIG. 19(d) illustrates a reproduced signal waveform obtained when the same track is subjected to reproduction by using a continuous light beam having a constant reproducing light power. Pr2 and Pr1 are selected as follows. That is, Pr2 is a recording power to cause the magnification of the magnetic domain in the auxiliary magnetic film 8 as described later on. Pr1 is a power to extinguish the magnified magnetic domain. When the reproducing power is selected as described above, the amplitude Hpl, which is provided between the recording state and the erasing state observed during the reproduction with the pulse light beam, is allowed to satisfy Hpl greater than Hdc with respect to the amplitude Hdc obtained upon the reproduction with the constant laser beam. Further, the magnetization information, which is recorded in each of the magnetic domains of the magneto-optical recording film, can be independently magnified and reproduced without being affected by adjacent magnetic domains.
An object of the present invention is to provide a reproducing method which is achieved by further improving the reproducing method disclosed in International Publication WO 97/22969.
The present invention has been made in order to solve the problems involved in the conventional technique, by means of a method different from the methods described in Japanese Laid-Open Patent Publication Nos. 1-143041 and 8-7350, an object of which is to provide a magneto-optical recording medium and a method for reproducing signals thereon, wherein a reproduced signal is obtained with sufficient C/N even when a minute magnetic domain is subjected to recording.
Another object of the present invention is to provide a magneto-optical recording medium and a reproducing method thereon which make it possible to reliably erase a magnified magnetic domain immediately after reproduction of a recording magnetic domain even when the magnetic domain is magnified during reproduction.
According to a first aspect of the present invention, there is provided a method for performing reproduction on a magneto-optical recording medium for reproducing a recorded signal by irradiating the magneto-optical recording medium with a reproducing light beam to detect magnitude of an magneto-optical effect, the reproducing method comprising the steps of:
using, as the magneto-optical recording medium, a magneto-optical recording medium comprising a magneto-optical recording film having perpendicular magnetization, an auxiliary magnetic film which transfers from an in-plane magnetizable film to a perpendicularly magnetizable film when a temperature exceeds a critical temperature Tcr and a non-magnetic film intervened between the magneto-optical recording film and the auxiliary magnetic film, the magneto-optical recording medium having a magnetic characteristic to satisfy a relationship of room temperature less than Tcr less than Tcomp less than Tco less than Tc concerning a Curie temperature Tco of the magneto-optical recording film and a Curie temperature Tc and a compensation temperature Tcomp of the auxiliary magnetic film; and
executing reproduction of the recorded signal through the steps of irradiating the magneto-optical recording medium with the reproducing light beam which is power-modulated to have at least two light powers of Pr1 and Pr2 at the same cycle as that of a reproducing clock or at a cycle created by the multiplication of an integer and the reproducing clock while applying a DC magnetic field so that a recording magnetic domain in the magneto-optical recording film is transferred to the auxiliary magnetic film, the transferred magnetic domain is magnified, and the magnified magnetic domain is reduced or extinguished, wherein:
the magneto-optical recording medium is specified, under a condition in which an external magnetic field Hex is applied to the magneto-optical recording medium, such that a temperature curve A of a transfer magnetic field which is generated by the external magnetic field Hex and the magneto-optical recording film, and a temperature curve B of a coercive force of the auxiliary magnetic film in a perpendicular direction intersect at a point between room temperature and the compensation temperature Tcomp of the auxiliary magnetic film, and the temperature curve A and the temperature curve B intersect at a point between the compensation temperature Tcomp of the auxiliary magnetic film and the Curie temperature Tco of the magneto-optical recording film.
It is preferable in the reproducing method of the present invention that the light power Pr1 of the reproducing light beam is a power to heat the auxiliary magnetic film to a temperature from Tcr to Tcomp so that the recording magnetic domain in the magneto-optical recording film is transferred to the auxiliary magnetic film and the magnetic domain is magnified, and the light power Pr2 of the reproducing light beam is a power to heat the auxiliary magnetic film to a temperature from Tcomp to Tco so that the magnified magnetic domain is reduced or extinguished.
The transfer magnetic field may be represented by a sum of the external magnetic field Hex and a static magnetic field Ht from the magneto-optical recording film, and the coercive force of the auxiliary magnetic film in the perpendicular direction may be represented by a sum of a coercive force Hr in the perpendicular direction of the magnetic domain subjected to the transfer and an exchange coupling force Hw exerted on the magnetic domain subjected to the transfer by adjoining magnetic domains.
The method for transferring the recording magnetic domain inscribed on the recording layer to the reproducing layer so that the transfer signal on the reproducing layer is magnified and read in order to obtain a high quality reproduced signal is called xe2x80x9cmagnetic amplification-mediated magneto-optical system (MAMMOS)xe2x80x9d, which has been confirmed by the present applicant by using the external magnetic field modulation reproducing method (WO 98-02878). In the external magnetic field modulation reproducing method, the magnification and the reduction are executed for the magnetic domain transferred to the reproducing layer by using an alternating magnetic field during reproduction. In the present invention, experiments have been carried out from various viewpoints for the magneto-optical system based on the magnetic amplification to advance detailed analysis and investigation. As a result, the present inventors have succeeded in development of the method which makes it possible to reliably realize the magnification and the reduction of the transferred magnetic domain by making modulation to give two or more kinds of reproducing light powers by using the direct current magnetic field.
Explanation will be made for the principle of the reproducing method on the magneto-optical recording medium according to the first aspect of the present invention. The reproducing method is based on the use of the magneto-optical recording medium comprising the magneto-optical recording film having the perpendicular magnetization, and the auxiliary magnetic film which causes transition from the in-plane magnetizable film to the perpendicularly magnetizable film when the temperature exceeds the critical temperature Tcr, with the non-magnetic film interposed therebetween. FIG. 9 shows an illustrative structure of the magneto-optical recording medium of this type. A magneto-optical disk 90 shown in FIG. 9 comprises, in a stacked form on a substrate 1, a dielectric film 3, an auxiliary magnetic film 8, a non-magnetic film 3, a magneto-optical recording film 10, and a protective film 7. The auxiliary magnetic film 8 has a compensation temperature Tcomp between a critical temperature Tcr and its Curie temperature Tc. The magneto-optical recording medium 90 satisfies the relationship of room temperature less than Tcr less than Tcomp less than Tco less than Tc concerning the Curie temperature Tco of the magneto-optical recording film 10, the critical temperature Tcr, the Curie temperature Tc, and the compensation temperature Tcomp of the auxiliary magnetic film 8.
Reproduction is performed in accordance with the reproducing method of the present invention by radiating the light power-modulated reproducing light beam while applying the external DC magnetic field to the magneto-optical recording medium 90 having the magnetic characteristic as described above. FIG. 11 shows magnetic characteristics of the magneto-optical recording film 10 and the auxiliary magnetic film 8 of the magneto-optical disk 90 in a state in which the constant DC magnetic field Hex is applied to the magneto-optical recording medium 90 in the recording direction. The magnetic temperature curve A shown in FIG. 11 denotes a temperature-dependent change in transfer magnetic field (static magnetic field) generated by the magnetization of the recording layer from the magneto-optical recording film 10 (hereinafter simply referred to as xe2x80x9crecording layerxe2x80x9d) to the auxiliary magnetic film 8 (hereinafter simply referred to as xe2x80x9creproducing layerxe2x80x9d). The transfer magnetic field of the curve A represents the magnitude of the magnetic field obtained by adding an amount of offset of the external magnetic field Hex. Therefore, the magnetic filed having the magnitude of (Hexxe2x88x92Ht) and the magnetic field having the magnitude of (Hex+Ht) exist as the entire transfer magnetic field depending on the direction of the magnetic domain of the recording layer, with a boundary of the Curie temperature Tco of the recording layer. The two magnetic fields constitute the curve A. In FIG. 9, the downward direction is the recording direction. Hex is applied in the downward direction. In this case, the external magnetic field Hex is adjusted to be small as compared with the magnitude of the static magnetic field Ht in the initializing direction generated from the magnetization of the recording layer at room temperature. Therefore, the entire transfer magnetic field includes those directed in the upward direction (negative) and in the downward direction (positive) depending on the magnetization direction of the recording magnetic domain in the recording layer as illustrated by the curve A.
The magnetic temperature curve B denotes the temperature-dependent change of the coercive force in the perpendicular direction of the reproducing layer in a state of having the perpendicular magnetization. The coercive force is represented by Hr+Hw as including the pure coercive force Hr of the magnetic domain in the reproducing layer in the perpendicular direction and the magnetic field Hw corresponding to a virtual magnetic field regarded to be applied by generation of the magnetic wall of the reproducing layer (in other words, the exchange coupling magnetic field in the in-plane direction of the reproducing layer). That is, Hr+Hw represents the magnetic field necessary to perform inversion of the magnetization in the direction perpendicular to the film surface of the reproducing layer. As shown in FIG. 11, the magnetization in the direction perpendicular to the film surface of the reproducing layer appears at a temperature which is not less than the critical temperature Tcr (T0 in FIG. 11) at which the reproducing layer behaves as a perpendicularly magnetizable film. The coercive force is maximal at the compensation temperature Tcomp because the magnetization of the reproducing layer is zero.
The temperature curves A and B shown in FIG. 11 are divided into those belonging to three areas (a) to (c) as shown in FIG. 11. The three areas (a) to (c) correspond to the three steps of i) magnetic domain transfer from the recording layer to the reproducing layer, ii) magnification of the transferred magnetic domain in the reproducing layer, and iii) extinguishment of the magnified magnetic domain, in the reproducing method of the present invention as shown in FIG. 12A respectively. Accordingly, explanation will be made with reference to FIG. 12 for the magnetic characteristics required for the recording layer and the reproducing layer in the areas (a) to (c) shown in FIG. 11. Arrows in the recording layer and the reproducing layer shown in FIG. 12A denote the direction of the magnetic moment of the rare earth metal included in each of the magnetic domains.
The area (a) is a temperature area in which the magnetic domain is transferred from the recording layer to the reproducing layer in the reproducing method of the present invention, which belongs to a temperature range of T0 to T1 in FIG. 12A. T0 means the critical temperature Tcr, and T1 is a temperature at which the magnetic temperature curve A on the side of Hexxe2x88x92Ht initially intersects the magnetic temperature curve B. The temperature range T0 to T1 can be achieved by adjusting the light power of the reproducing light beam to be a relatively low power as described later on. In order to actually perform the magnetic transfer as shown in FIG. 12A (1) in this temperature area, it is necessary that the magnitude of the transfer magnetic field in this temperature area exceeds the coercive force of the reproducing layer in the perpendicular direction. That is, when the magnetization recorded on the recording layer is in the direction ↓ (recording direction), it is necessary that the transfer magnetic field represented by Hex+Ht is larger than Hr+Hw or xe2x88x92(Hr+Hw) (requirement for magnetic domain transfer). When the magnetization recorded on the recording layer is in the direction ↑ (erasing direction), it is necessary that the negative transfer magnetic field represented by Hexxe2x88x92Ht is smaller than the coercive force Hr+Hw or xe2x88x92(Hr+Hw) of the reproducing layer in the perpendicular direction (requirement for magnetic domain transfer).
On the other hand, when the magnetic temperature curves A and B are compared with each other in the area (a) shown in FIG. 11, it is appreciated that the relationships of the following expressions (a1) to (a3) hold.
Hr less than Hex+Htxe2x88x92Hwxe2x80x83xe2x80x83(a1)
xe2x88x92Hr greater than Hexxe2x88x92Ht+Hwxe2x80x83xe2x80x83(a2)
Hr greater than Hexxe2x88x92Htxe2x88x92Hwxe2x80x83xe2x80x83(a3)
Therefore, the area (a) satisfies the magnetic domain transfer requirement described above, and the recording magnetic domain in the recording layer can be transferred to the reproducing layer regardless of the direction of magnetization thereof. FIG. 12A (1) shows a case in which the magnetization in the direction ↓ recorded in a magnetic domain 210 in the recording layer is transferred to an area of the reproducing layer at a temperature which exceeds the temperature T0 within the reproducing light spot so that a transferred magnetic domain 201a is formed.
Subsequently, in the area (b) shown in FIG. 11, the magnetic domain magnification is performed for the magnetic domain 201b transferred to the reproducing layer as shown in FIG. 12A (2) and (3). This temperature area resides in a range indicated by T1 to T2 in FIG. 11. The temperature T2 is a temperature at which the magnetic temperature curve A on the side of Hexxe2x88x92Ht intersects the magnetic temperature curve B on the high temperature side. The magneto-optical disk having the magnetic characteristic shown in FIG. 11 is adjusted such that T2 is approximately coincident with the compensation temperature Tcomp of the reproducing layer (the temperature exists between the compensation temperature Tcomp and the Curie temperature Tco of the recording layer, and the temperature is a temperature extremely close to the compensation temperature Tcomp) in relation to the external magnetic field Hex. In this temperature area, as shown in FIG. 12A (2), magnetic domains 203, 203xe2x80x2, which are subjected to magnetic transfer from magnetic domains 212, 212xe2x80x2 in the recording layer in the upward direction, exist on both sides of the magnetic domain 201b transferred to the reproducing layer, as a result of being heated to T0 to T1 within the reproducing light spot. In order to allow the magnetic domain 201b transferred to the reproducing layer to start magnification in the in-plane direction, it is necessary that the directions of the magnetic domains 203, 203xe2x80x2 disposed on the both sides are directed to the recording direction (direction ↓) in the same manner as the magnetic domain 201b. The magnetic domains 203, 203xe2x80x2 receives the transfer magnetic field (Hexxe2x88x92Ht) (totally in the direction ↑) obtained by adding, to the external magnetic field Hex, the static magnetic field Ht in the upward direction from magnetic domains 212 in the recording layer existing just thereover. On the other hand, the magnetic domains 203, 203xe2x80x2 have the coercive force in the perpendicular direction including the exchange coupling magnetic field Hw (in the downward direction) exerted by the magnetic domain 201b and the coercive force Hr to invert the magnetization of the magnetic domains 203, 203xe2x80x2 themselves. Therefore, when the coercive force in the perpendicular direction (Hr+Hw) is made larger than the transfer magnetic field (Hexxe2x88x92Ht) of the magnetic domains 203, 203xe2x80x2, the magnetic domains 203, 203xe2x80x2 are inverted (requirement for magnetic domain inversion).
It is appreciated that the following relational expressions hold in the area (b) according to the relative magnitude between the magnetic temperature curves A and B.
Hr less than Hex+Htxe2x88x92Hwxe2x80x83xe2x80x83(b1)
xe2x88x92Hr less than Hexxe2x88x92Ht+Hwxe2x80x83xe2x80x83(b2)
Hr greater than Hexxe2x88x92Htxe2x88x92Hwxe2x80x83xe2x80x83(b3)
The foregoing expression (b2) is the condition of magnetic domain inversion itself under which the coercive force (Hr+Hw) in the perpendicular direction is larger than the transfer magnetic field Hexxe2x88x92Ht (in the upward direction) of the magnetic domains 203, 203xe2x80x2. Therefore, the magnetic domain magnification occurs in the area (b) for the magnetic domain 201b in the reproducing layer as shown in FIG. 12A (3). According to the relationship of (b2), it is demonstrated that no magnetic domain in the downward direction appears in the reproducing layer when there is no magnetic domain in the recording direction in the reproducing layer, in the temperature area (b). In FIG. 12A (3), the both sides of the magnified magnetic domain 201b are the temperature area of T0 to T1. Therefore, the magnetic domains 203, 203xe2x80x2 in the direction ↑, which are subjected to the magnetic domain transfer from the magnetic domains 212, 212xe2x80x2 in the recording layer, exist therein.
Subsequently, in the area (c), the transferred and magnified magnetic domain is inverted (extinguished), and a magnetic domain 201c in the erasing direction is formed as shown in FIG. 12A (4). This temperature area exists in a range from T2 which slightly exceeds the compensation temperature of the reproducing layer, to the Curie temperature Tco of the recording layer. The magnified and reproduced magnetic domain can be extinguished or reduced by applying the reproducing magnetic field in the erasing direction, i.e., by using the alternating magnetic field as the reproducing magnetic field. However, in the reproducing method of the present invention, the DC magnetic field is used to extinguish the magnified magnetic domain by power-modulating the reproducing light beam to have the power higher than the reproducing light power used to perform the magnetic transfer and the magnification. The reproducing light power may be modulated to be further small in order to extinguish the magnified magnetic domain, as described in the first embodiment of the reproducing method on the magneto-optical recording medium according to the present invention as described later on.
Explanation will be made with reference to FIGS. 13A and 13B for the principle to invert (extinguish) the magnified magnetic domain in the area (c). FIGS. 13A and 13B illustrate the temperature-dependent change of the direction and the magnitude of sub-lattice magnetization of the rare earth metal and the transition metal of the magnetic domain 210 in the recording layer composed of the rare earth-transition metal (TbFeCo alloy) and the magnetic domain 201b in the reproducing layer composed of the rare earth-transition metal (GdFeCo alloy) subjected to the magnetic domain transfer therefrom shown in FIG. 12A (2). As shown in FIG. 13A, when the temperature of the reproducing layer is less than the compensation temperature Tcomp, then the magnetization of the rare earth metal in the reproducing layer is dominant, and it is parallel to the magnetization direction of the recording layer of the transfer source (the magnetization of the transition metal is dominant). Subsequently, when the temperature of the reproducing layer exceeds the compensation temperature Tcomp by radiating the high power laser in accordance with the reproducing method of the present invention, the magnetic moment of the transition metal in the reproducing layer is dominant. It is appreciated that the following expressions (c1) and (c2) hold according to the relative magnitude of the magnetic temperature curves A and B of the reproducing layer and the recording layer in the area (c) shown in FIG. 11.
Hr less than Hex+Htxe2x88x92Hwxe2x80x83xe2x80x83(c1)
Hr less than Hexxe2x88x92Htxe2x88x92Hwxe2x80x83xe2x80x83(c2)
That is, the coercive force Hr of the magnetic domain 201b is smaller than the entire magnetic field (Hex+Htxe2x88x92Hw or Hexxe2x88x92Htxe2x88x92Hw) in the recording direction acting on the magnetic domain 201b. As a result, when the temperature of the reproducing layer is not less than the compensation temperature Tcomp (exactly, when it is not less than T2), the dominant magnetic moment of the transition metal is inverted to be directed in the recording direction as shown in FIG. 13B. Therefore, the magnetic moment of the rare earth metal in the downward direction of the magnified magnetic domain 201b shown in FIG. 12A (3) is inverted in the area which is heated to the temperature not less than the temperature of the area (c), i.e., not less than the compensation temperature Tcomp. Thus, the inverted magnetic domain 201c is generated (FIG. 12A (4)). The magnetic domains 201d, 201dxe2x80x2, which are disposed on the both sides of the inverted magnetic domain 201c, have their temperatures ranging from T1 to T2. Therefore, the magnetic domains 201d, 201dxe2x80x2 have the same magnetization direction as that of the magnified magnetic domain 201b. 
In the reproducing method according to the present invention, the three temperature areas (a) to (c) can be achieved by modulating the reproducing light power to have at least the two power levels Pr1 and Pr2 as shown in FIG. 12B. That is, the light power Pr1 of the reproducing light beam may be the power for heating the auxiliary magnetic layer to the temperature of Tcr to Tcomp and making it possible to transfer the recording magnetic domain in the magneto-optical recording film to the reproducing layer and magnify the magnetic domain. The light power Pr2 of the reproducing light beam may be the power for heating the auxiliary magnetic layer to the temperature of Tcomp to Tco and reducing or extinguishing the magnified magnetic domain as described above. The Pr1/Pr2 power-modulated reproducing light beam is used as the reproducing light beam in synchronization with the reproducing clock. Thus, the recording magnetic domain in the recording layer can be subjected to reproduction through the steps of i) transfer to the reproducing layer, ii) magnification of the transferred magnetic domain, and iii) extinguishment of the magnified magnetic domain.
According to a second aspect of the present invention, there is provided a magneto-optical recording medium having at least a magneto-optical recording film on a substrate, the magneto-optical recording medium comprising the magneto-optical recording film having perpendicular magnetization, an auxiliary magnetic film which transfers from an in-plane magnetizable film to a perpendicularly magnetizable film when a temperature exceeds a critical temperature Tcr with a non-magnetic film intervened between the magneto-optical recording film and the auxiliary magnetic film, wherein a relationship of room temperature less than Tcr less than Tcomp less than Tco less than Tc holds concerning a Curie temperature Tco of the magneto-optical recording film and a Curie temperature Tc and a compensation temperature Tcomp of the auxiliary magnetic film, and wherein under a condition in which an external magnetic field Hex is applied to the magneto-optical recording medium, a temperature curve A of a transfer magnetic field which is generated by the external magnetic field Hex and the magneto-optical recording film, and a temperature curve B of a coercive force of the auxiliary magnetic film in a perpendicular direction intersect at a point between room temperature and the compensation temperature Tcomp of the auxiliary magnetic film, and the temperature curve A and the temperature curve B intersect at a point between the compensation temperature Tcomp of the auxiliary magnetic film and the Curie temperature Tco of the magneto-optical recording film.
The magneto-optical recording medium according to the second aspect of the present invention is a magneto-optical recording medium which is preferably used for the reproducing method according to the first aspect of the present invention. Even in the case of a minute magnetic domain which is smaller than the light spot, the magnetic domain can be subjected to reproduction independently from other magnetic domains to give an amplified reproduced signal, by performing reproduction on the magneto-optical recording medium by using the reproducing method according to the first aspect of the present invention. It is preferable that the temperature T2, at which the temperature curve A and the temperature curve B intersect, satisfies Tcompxe2x89xa6T2xe2x89xa6Tco.
According to a third aspect of the present invention, there is provided a magneto-optical recording medium having at least a magneto-optical recording film on a substrate, the magneto-optical recording medium comprising:
a first auxiliary magnetic film which causes transition from a perpendicularly magnetizable film to an in-plane magnetizable film when a temperature exceeds a critical temperature Tcr11; and
a second auxiliary magnetic film which causes transition from an in-plane magnetizable film to a perpendicularly magnetizable film when the temperature exceeds a critical temperature Tcr12.
Explanation will be made with reference to FIG. 14 for an example of the structure of the magneto-optical recording medium according to the third aspect of the present invention. As shown in FIG. 14, the magneto-optical recording medium 100 successively comprises, on a magneto-optical recording film 10, a first auxiliary magnetic film 28, a non-magnetic film 29, and a second auxiliary magnetic film 24. The magneto-optical recording film 10 is a perpendicularly magnetizable film. The first auxiliary magnetic film 28 is a magnetic film which causes transition from a perpendicularly magnetizable film to an in-plane magnetizable film when the temperature exceeds the critical temperature Tcr11. The second auxiliary magnetic film 24 is a magnetic film which causes transition from an in-plane magnetizable film to a perpendicularly magnetizable film when the temperature exceeds the critical temperature Tcr12. It is assumed herein that materials and compositions of the magnetic films are adjusted so that the critical temperature Tcr11 of the first auxiliary magnetic film is higher than the critical temperature Tcr12 of the second auxiliary magnetic film. The second auxiliary magnetic film 24 functions as a reproducing layer.
Explanation will be made with reference to FIGS. 16A to 16C for the principle of reproduction on the magneto-optical recording medium according to the third aspect. FIG. 16A conceptually illustrates main components of the magneto-optical recording medium shown in FIG. 14. It is assumed that the magnetization in the upward direction is recorded in a magnetic domain 22 of the magneto-optical recording film 10. The magneto-optical recording film 10 and the first auxiliary magnetic layer 28 make exchange coupling to one another. The same magnetization as that of the magnetic domain 22 is transferred to a magnetic domain 28a of the first auxiliary magnetic layer 28 disposed just under the magnetic domain 22. When the magneto-optical recording medium is irradiated with a reproducing light means, and the temperature begins to rise, then the transition occurs from the in-plane magnetization to the perpendicular magnetization in an area of the second auxiliary magnetic film 24 in which its temperature exceeds the critical temperature Tcr12. The area subjected to the transition corresponds to magnetic domains 24a, 24b shown in FIG. 16B. During the transition, the magnetic domain 24a is aligned in the same magnetization direction as that of the magnetic domain 22 as shown in FIG. 16B by the aid of the magnetostatic coupling force exerted by the magnetic domain 22 of the recording layer 10 disposed just thereover and the magnetic domain 28a of the first auxiliary magnetic film 28. FIG. 16B illustrates the temperature-rising process of the magneto-optical recording medium effected by the reproducing light beam, and it represents a magnetization state in which the temperature T of the magneto-optical recording medium does not arrive at a maximum arrival temperature yet and the temperature is within a range of Tcr12 less than T less than Tcr11. In this state, the recording layer 10, the first auxiliary magnetic layer 28, and the second auxiliary magnetic layer 24 are magnetically coupled (magnetostatically coupled) to one another, and any of them exhibits the perpendicular magnetization. Minute magnetic domains 24b, which have the magnetization in the downward direction by the aid of the magnetostatic coupling force exerted by the both magnetic domains adjacent to the magnetic domain 22 and the magnetic domains in the downward direction in the first auxiliary magnetic film 28 disposed just thereunder, are present on both adjoining sides of the magnetic domain 24a. 
When the temperature of the medium is further raised to arrive at the heating maximum temperature, if the temperature of the high temperature area of the first auxiliary magnetic layer 28 exceeds the critical temperature Tcr11, then the coercive force of the first auxiliary magnetic layer 28 is lowered, and thus the first auxiliary magnetic layer 28 in the high temperature area causes transition from the perpendicular magnetization to the in-plane magnetization. As a result, a magnetic domain 28axe2x80x2 is formed as shown in FIG. 16C.
FIG. 17 shows a relationship between the temperature distribution and the magnetization state of the medium shown in FIG. 16C. In the case of this magneto-optical recording medium, there is given Tcr12 less than Tcr11 as described above. Accordingly, as shown in FIG. 17, the area, in which the temperature exceeds Tcr12 in the temperature distribution of the medium, is wider than the area in which the temperature exceeds Tcr11. The transition occurs from the in-plane magnetization to the perpendicular magnetization in the area in which the temperature exceeds Tcr12 in the second auxiliary magnetic layer 24. The transition occurs from the perpendicular magnetization to the in-plane magnetization in the area in which the temperature exceeds Tcr11 in the first auxiliary magnetic layer 24. Therefore, the magnetic domain 24axe2x80x2 having the perpendicular magnetization in the second auxiliary magnetic layer 24 is larger than the magnetic domain 28axe2x80x2 having the in-plane magnetization in the first auxiliary magnetic layer 24. The reproducing light power and Tcr12 are adjusted so that the area, in which the temperature exceeds Tcr12 in the second auxiliary magnetic layer 24 upon irradiation with the reproducing light beam, is larger than the magnetic domain in the recording layer 10.
On the other hand, the magnetic domain 28axe2x80x2 in the first auxiliary magnetic layer 28 has the in-plane magnetization. Therefore, the magnetic influence can be intercepted, which would be otherwise exerted from the magneto-optical recording film 10 to the second auxiliary magnetic film 24, due to, for example, the leakage magnetic field and the static magnetic field caused by the magnetization in the direction ↓ existing on both adjoining sides of the magnetic domain 22. Accordingly, it is possible to facilitate the magnification of the magnetic domain 24axe2x80x2. The magnification of the magnetic domain increases the reproduced signal. It is considered that C/N is improved owing to the function of the first auxiliary magnetic film 24 to cause magnetic interception. In order to more effectively use the magnetically intercepting function of the first auxiliary magnetic film 28, it is preferable that the critical temperature Tc11 of the first auxiliary magnetic film 28 and the reproducing light power are selected so that the area, in which the temperature exceeds Tcr11 in the first auxiliary magnetic layer 28 during reproduction, is larger than the recording magnetic domain 11. In order to obtain a sufficiently large reproduced signal by the aid of the magnetic domain magnification in the second auxiliary magnetic layer 24, it is preferable that the critical temperature Tc12 of the second auxiliary magnetic film 24 and the reproducing light power are selected so that the area, in which the temperature exceeds Tcr12 in the second auxiliary magnetic layer 24 during reproduction, is larger than the recording magnetic domain 11. In order to simultaneously satisfy the facilitating effect for magnifying the magnetic domain and the magnetically intercepting function of the first auxiliary magnetic film 28, it is desirable to appropriately control the relationship (xcex94T=Tcr11xe2x88x92Tcr12) between the critical temperature Tcr11 of the first auxiliary magnetic film 28 and the critical temperature Tcr12 of the second auxiliary magnetic film 24.
The effect of the magnification of the magnetic domain of the second auxiliary magnetic film 24, i.e., the reproduced signal intensity is maximized when the transferred magnetic domain in the second auxiliary magnetic film 24 is magnified to be not less than the reproducing light spot diameter. In this state, an extremely large reproduction output, which is determined by only the performance index of the second auxiliary magnetic film 24 and the reproducing light beam, is obtained regardless of the size and the shape of the magnetic domain recorded in the magneto-optical recording film 10. After the reproduction, i.e., after the unit for radiating the reproducing light beam is moved, the readout portion is cooled to be not more than Tcr12, and the second auxiliary magnetic film is in the in-plane magnetization state to return to the state shown in FIG. 16A. The coercive force of the magneto-optical recording film 10 is sufficiently large even at the temperature during the reproducing operation as described above. Therefore, the information recorded as magnetization is completely retained.
It is desirable for the magneto-optical recording medium according to the third aspect of the present invention that a relationship of room temperature less than Tcr12 less than Tcr11 less than Tco, Tc1, Tc2 holds concerning a Curie temperature Tco of the magneto-optical recording film, a Curie temperature Tc1 and the critical temperature Tcr11 of the first auxiliary magnetic film, and a Curie temperature Tc2 and the critical temperature Tcr12 of the second auxiliary magnetic film.